1. Field of the Invention
The present invention relates to a method and apparatus for processing a digital image signal acquired by an image sensing device, such as a digital camera, which records a digital image signal.
2. Description of the Related Art
Recently, digitization is progressing in various fields, and system configurations are being changed substantially. Photograph-related industries are not an exception, and digital still cameras (hereinafter called digital cameras) which photoelectrically convert object images into image signals by using CCDs, and digitize and record the image signals by AD conversion thereon have been spreading.
Following the spread of such digital cameras, a service (hereinafter called a digital printing service) for generating color prints from digital image signals acquired by digital cameras is gradually spreading. However, the image quality of prints obtained by using such a service is inferior to that by a conventional color printing service (hereinafter called a conventional service) wherein prints are generated from negative color films. As factors of this, the following can be listed:
1) Image quality of a digital camera itself as an image sensing device
2) Differences in performance between different types of digital cameras
3) Differences in performance between digital cameras of the same type
4) Differences or fluctuation in performance of a digital camera
For the image quality of digital cameras listed in 1), it is rapidly improving due to enhanced performance of CCDs or the like. However, for the performance differences in 2) to 4), no measures have been taken.
Meanwhile, in a conventional service, the same problems as in the digital printing service exist. Changing the 4 factors in the digital printing service into those in a conventional service, the following is listed:
1′) Image quality of a negative color film itself and that of a camera using silver halide films as an image sensing device
2′) Differences in performance between varieties of negative color films
3′) Differences in performance due to various factors (such as development processing, temperature, and humidity) related to negative color films of the same variety
4′) Differences in performance due to various factors of a negative color film
In a conventional service, color balance and print density are the only parameters controlling the image quality. Especially, the color balance is the most important parameter. Therefore, in order to improve print quality, various methods for stabilizing the color balance have been proposed.
For example, in order to cancel the difference in print quality due to the factor in 2′), a method wherein characteristic information regarding a negative color film is stored in a memory or the like for each type of negative film and a print generating condition is decided based on the information has been proposed (this is a so-called multi-channel method). However, in the case of a negative color film, the factors in 3′) and 4′), such as a difference in development processing and a performance change due to temperature and humidity, are very influential. Especially, once performance of a negative color film has changed due to exposure to high temperature or the like, the performance does not recover. Therefore, the improvement of print quality by the multi-channel method has a limit. For this reason, in the case where an image in each frame is read from a film having a plurality of frames and print exposure is decided based on primary-color information of each pixel in each frame, a method of controlling the exposure has been proposed (see Japanese Unexamined Patent Publication No. 55(1980)-46741). In this method, density differences between a first and a second primary colors, between a third primary color and either the first or the second primary color, and the average density of the 3 primary colors are found to generate a characteristic curve representing relations of the density differences to the average density, and exposure is controlled by finding a characteristic value for the negative color film based on the characteristic curve. As has been described in the above, in the case of a negative color film, the above factors 3′) and 4′) are influential, and it is necessary to use information of a plurality of frames in order to estimate the characteristic of a negative film even when the films are of the same variety.
Meanwhile, in the case of a digital camera, since tens of new types are released every year, it is practically impossible to decide a print generating condition by using characteristic information of each type of digital cameras, as in the multi-channel method. Therefore, it is necessary to estimate, using some method, the performance differences stated in 2) between various types of digital cameras.
Meanwhile, CCDs used in digital cameras are very durable, and have little performance change when the temperature or humidity changes. For example, under a high temperature, noise due to a dark current increases, while signal strength shows little change. Unlike negative films, the level of the dark current resumes when the temperature is changed to the original temperature. Since CCDs have little performance change as has been described in the above, one image signal has been thought to be enough to estimate the characteristic of a digital camera, and little print quality improvement has been expected from usage of a plurality of image signals to estimate the characteristic of the digital camera.
However, in a digital printing service, even when the difference in 2) above can be suppressed completely, the differences in 3) and 4) are not negligible. In other words, even when the types of digital cameras are the same, or when the performance differences due to the types of digital cameras can be suppressed, color differences due to variances between digital cameras, such as subtle differences in sensitivity of CCDs or differences in lens performances can be created. Furthermore, in a digital camera having an automatic white-balancing function, image signals can have variances due to a difference based on each camera, and the print quality may be lowered as a result of this.